1. Field of the Invention
The present invention relates to a frame for a vehicle such as a bicycle. More particularly, the invention relates to a bicycle frame constructed from two, thin-walled cast shells joined together to form a unitary monocoque frame.
2. Description of the Related Art
Traditionally, bicycle frames are formed from hollow steel tubes that are connected together to form a two triangle design. A front or main triangle includes a top tube, a down tube, a head tube, a seat tube, and a bottom bracket. A rear triangle includes a pair of seat stays, a pair of chain stays, and a pair of rear axle dropouts. The tubes are connected together, such as by brazing, to form the bicycle frame. The high compressive strength characteristics of steel tubes, their availability and low cost, and their formability, make them ideally suitable for the two triangle design, such that many manufacturers still use it with minor variations in frame geometry.
An essential feature of this design, however, is that all the tubes must be connected together to form the finished frame, so that formation of the frame from tubes is labor intensive. Further, the lugs or joints with which the tubes are connected must be relatively thick and hence heavy so as to provide an adequately strong connection. Another disadvantage of steel frames is the need for corrosion protection, requiring numerous additional processing steps such as spraying the frame with primer, then paint, and allowing suitable drying time in between steps.
More recently, other materials have become more readily available, such as aluminum, magnesium, titanium, and fiber-reinforced plastic composites, allowing manufacturers the ability to construct bicycle frames with reduced weight. Many manufacturers have merely substituted tubes made of these materials for steel tubes, and have employed welding or gluing of the frame joints instead of brazing in the traditional two triangle design. However, the formation of these frames remains time consuming, and in some cases, the frame is not aesthetically pleasing.
A bicycle frame that avoids many of the above-mentioned disadvantages of conventional tube frames is formed from two half shells that are bonded together to form a unitary monocoque frame. An example is shown in U.S. Pat. No. 5,464,240, which describes a hollow bicycle frame constructed from composite injection molded partial shells that are joined together using numerous protrusions and receptacles disposed along joining faces of each partial shell. These protrusions and receptacles are very small because of the wall thickness of the partial shells, and are thus relatively difficult to manufacture without defects. The protrusions may easily break before or during joining. Further, alignment of the partial shells requires an additional step of aligning the protrusions with their corresponding receptacles, which is time consuming and hence labor intensive.
There have been numerous other attempts to replace the conventional tube frame with a frame in the form of a hollow shell of formed or stamped sheet metal, molded fiberglass or the like. However, none of these attempts have been widely adopted, partially due to the fact that they did not have an adequate strength-to-weight ratio, and in particular they did not have adequate torsional rigidity, even though they were heavier than the conventional tube frame in many cases. Examples of prior U.S. patents disclosing hollow shell bicycle frames having these deficiencies are U.S. Pat. Nos. 3,233,916, 3,375,024, 4,230,332 and 4,613,146.
In view of the above disadvantages, it would therefore be desirable to provide a unitary monocoque frame formed from two half shells including a structure for joining the two half shells that is relatively simple and not labor intensive.